docs: def scale(self, selections, factor):
          '''Scale the objects represented by *selections* up to a
          certain *factor*.
  
          '''
          if 'atoms' in selections:
              atms = selections['atoms'].mask
              if factor is None:
                      self.scale_factors[atms] = 1.0
              else:
                  self.scale_factors[atms] = factor
      
          self.update_scale_factors(self.scale_factors)